Recombining separated gaseous nitrogen and vaporized liquid nitrogen with air to produce a constant gaseous feed rate



June 27, 1967 GREMER 3,327,490

RECOMBINING SEPARATED GASEOUS NITROGEN AND VAPORIZED LIQUID NITROGEN WITH AIR T0 PRODUCE A CONSTANT GASEOUS FEED RATE Filed Sept. 14, 1964 2 Sheets-Sheet l 4440/9/05 Gzz/vme June 27, 1967 GRENlER 3,327,490

RECOMBINING SEPARATED GASEOUS NITROGEN AND VAPORIZED LIQUID NITROGEN WITH AIR TO PRODUCE A CONSTANT GASEOUS FEED RATE Filed Sept. 14, 1964 2 Sheets-Sheet 2 [XPANOE/P United States Patent RECOMBINING SEPARATE!) GASEOUS NITROGEN AND VAPORIZED LIQUID NITROQEN WITH AIR 2TR0DUCE A CONSTANT GASEOUS FEED Maurice Greuier, Paris, France, assignor to LAir Liquide, Societe Anonyme Pour lEtude et lExploitation des Procedes Georges Claude Filed Sept. 14, 1964, Ser. No. 396,162 Claims priority, application France, Sept. 23, 1963, 948,348, Patent 1,380,546 3 Claims. (Cl. 6230) This invention relates to the production of low-temperatures or cold by the vaporisation of liquid nitrogen.

It has already been proposed to employ nitrogen vaporised at a low temperature for the production of cold, for example with the object of maintaining a vacuum in a chamber by freezing the gas penetrating thereinto on a surface cooled by the vaporisation of liquid nitrogen (an arrangement known as a cryopump) or of preserving products which may deteriorate at ambient temperature, by reheating it to a temperature in the vicinity of ambient temperature by heat exchange with nitrogen already reheated and compressed to a relatively high pressure, the latter being reliquefied after it has been cooled, in accordance with the Claude cycle principle, by heat exchange with a fraction expanded with external work followed by expansion to a low pressure.

However, this arrangement necessitates the availability of excesses of nitrogen in order to ensure the initial starting operation and to compensate for the inevitable losses, particularly during the vaporisation. If nitrogen excesses of external origin are considered satisfactory, the operation of the installation for the production of cold is dependent on the deliveries of nitrogen. On the contrary, if it is desired to have the nitrogen available on the spot, it is necessary to add, to the nitrogen liquefaction installation, an installation for the fractionation of air by liquefaction and rectification, this leading to the use of complex and costly equipment. At the same time, it is still only possible to ensure a substantially constant production of cold under good conditions and it is only possible to conform to a variable cold consumption if the capacity of the different installations is calculated so as to correspond approximately to the maximum instantaneous cold consumption which may be required.

It has been suggested that it would be possible to obtain a greater flexibility in the production of cold by producing the liquid nitrogen by fractionation of air by liquefaction and rectification. The liquid nitrogen so produced could then be stored in a heat-insulated reservoir, from which it could be sent for vaporisation at .a rate corresponding to the desired cold production. However, such a solution would be costly from the point of view of energy consumption, because of the necessity of providing the air-fractionation work.

It is an object of the present invention to obviate the disadvantages of the aforementioned two processes by providing a process providing a cold output which is variable Within wide limits, with a low energy consumption, while not being dependent on the supply of gas of external origin for the operation and the compensation of losses.

According to the present invention there is provided a process for the production of cold by the vaporisation of liquid nitrogen, in which nitrogen vapour at low temperature, obtained by the vaporisation of liquid nitrogen, is reheated to a temperature in the region of ambient temperature, at least in part by indirect heat exchange with a gas which has a high nitrogen content and from which said liquid nitrogen is to be produced, characterised in that (a) said gas having a high nitrogen content 3,327,496 Patented June 27, 1967 is formed by the admixture of air, gaseous nitrogen separated from said gas with a high nitrogen content, and at least a part of any nitrogen vapour obtained by the vaporisation of said liquid nitrogen for the production of cold and reheated by heat exchange with the said gas having a high nitrogen content, said mixture being supplied at a constant or substantially constant delivery rate and the proportion of said nitrogen vapour in the gas with a high nitrogen content increasing or decreasing as the production of cold increases or decreases; and (b) said gas having a high nitrogen content is separated by liquefaction and rectification into liquid nitrogen, gaseous nitrogen and gas having a low nitrogen content.

The process of the invention assures a close connection between the consumption of cold and the production of liquefied gas, such that any increase in the former necessarily causes an increase of the latter. It also permits the use of an installation which is only of moderate size, corresponding substantially to the mean cold consumption, and which is consequently much less costly than an installation corresponding to the desired maximum instantaneous cold production.

Advantageously, when the rectification of the gas having a high nitrogen content is effected in at least two rectification columns under diiferent pressures, which are preferably in heat exchange with one another through a combined condenser and vaporiser, the process of the invention is modified in either one of or both of the following ways:

(a). The delivery of reflux liquid to the head of the rectification column at the lower pressure is reduced when the deli-very of nitrogen vapour from the liquefied nitrogen vaporised for the production of cold increases and is shut off when the rate of flow of said nitrogen vapour is substantially equivalent to the rate of flow of separated liquid nitrogen; and

(b) The refluxed liquid sent to the head of the rectification column at thes lower pressure is taken from the rectification column at the higher pressure, appreciably under the top thereof.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in Which:

FIGURE 1 shows schematically an installation for the production of low temperatures by vaporisation of liquid nitrogen;

FIGURE 2 shows schematically a modified arrangement of that part of the installation shown in FIGURE 1 which is disposed on the right of the axis XX in FIG- URE l; and

FIGURE 3 shows schematically another embodiment of an installation for the production of low temperatures.

Referring first to FIGURE 1 of the accompanying drawing, atmospheric air, which is introduced through the pipe 1 (in the direction of the full-line arrows), is dried and the carbon dioxide removed therefrom at 2, for example by adsorption at ambient te-meprature on an adsorbent mass, e.g. activated alumina, in accordance with the process described in US. patent application Ser. No. 211,370 of July 20, 1962; the carbon dioxide may also be removed by washing with a sodium hydroxide solution, the drying then taking place, after the subsequent compression, by passing over an adsorbent mass. The air then has added thereto, through a pipe 3, gaseous nitrogen at or in the neighbourhood of ambient temperature, the gaseous nitrogen originating on the one hand from gaseous nitrogen separated by rectification and on the other hand from nitrogen vaporised to ensure the production of cold. The resulting gas with a high nitrogen content is then brought by a compressor 4 to a pressure of approximately 200 bars absolute, and is then passed through a pipe 5 into a heat exchanger 6, where it is cooled to about 25 C. by heat exchange with the cold nitrogen separated by rectification or vaporised in the cold consumption installation. A part of the cooled gas is then sent through a pipe 7 to an expansion machine 8 to about 6 bars absolute, and is blown through a pipe 9 into the bottom of a rectifier column 10 which is at a higher pressure. The other part of the cooled gas is passed through a pipe 11 into a heat exchanger 12, .in which it is cooled to a low temperature in heat exchange with separated oxygen and nitrogen vapours; it is then passed through a pipe 13 to a coil 14 disposed in the sump of the rectifier column 10 in which it is super-cooled; it is then passed through a pipe 15 into a heat exchanger 16, in which it is again supercooled by heat exchange with separated gaseous nitrogen; after passing through duct 17 it is then expanded through valves 18 and 19 and introduced into the high-pressure rectifier column 19. The respective rates of flow of liquefied gas passing through the valves 18 and 19 are regulated as a function of the cold production in a device 35, as will be explained hereinafter.

As in a conventional apparatus for rectification of air, the gas having a high nitrogen content is separated in the column 10 at a high pressure into an oxygen-enriched liquid and into a nitrogen-enriched liquid. The oxygenenriched liquid is sent through a pipe 22 to a heat exchanger 23, super-cooled in the latter by heat exchange With separated gaseous nitrogen, then introduced through a pipe 24 and an expansion valve 25 at about 1.3 bars absolute into the middle Zone of a low-pressure rectification column 21.

Liquid nitrogen, which is drawn off through a pipe 26 at the top of the high-pressure rectifier column 10, is super-cooled in a heat exchanger 27 in counter-current with the gaseous nitrogen leaving the upper end of the low-pressure rectification column 10, and then divided into two parts. The first part is introduced through a pipe 29 and an expansion valve 30 at 1.3 bars absolute to the head of the low-pressure rectification column 21 as reflux liquid. The second part constitutes the liquid nitrogen which is intended to ensure the production of cold, and is sent through a pipe 31 and an expansion Valve 32 in the region of atmospheric pressure into a heat-insulated reservoir 33. The heat-insulated reservoir 33 is equipped with a withdrawal valve 34 which connects it to a heat exchange device, represented diagrammatically by a coil 36, placed in a chamber 35 in which it is desired to bring about the production of cold. A pipe 37 permits the cold nitrogen vapours formed in the heat exchange device 36 to be sent back to the liquefaction device for the nitrogen-enriched gas. This recovery cannot be complete (as is the case, for example, when the liquid nitrogen is used for cooling products by direct contact); a line 37A represents the losses of vaporised gaseous nitrogen. On the other hand, a pipe 38 connected to the upper part, of the heat-insulated reservoir 33 enables the cold nitrogen vapours formed in this reservoir to be recovered and for them to be recombined With the vapours formed in the heat exchange device 36.

The nitrogen vapours are then sent through a pipe 39 into the heat exchanger 12, where they are reheated in counter-current with the nitrogen-enriched gas, and then through a conduit 40 into the heat exchanger 6, where they are reheated to the region of ambient temperature in counter-current with the same gas. They are then combined by means of the pipe 3 with the purified air at the intake of the compressor.

At the same time, the gaseous nitrogen separated at the head of the low-pressure rectifier column 21 passes successively through pipes 41, 42 and 43 into the exchangers 27, 23 and 16, respectively, in which it ensures the supercooling of the liquid separated in the high-pressure column 10 and of the nitrogen-enriched gas to be sepa- 4 rated. It is then combined by means of a pipe 44 with the cold nitrogen vapours at the cold end of the exchanger 12. Finally, the gaseous oxygen separated in the low-pressure column 21 is sent successively through pipes 45 and 46 into the exchangers 12 and 6, then discharged through a pipe 47.

The operation of the above-described installation as a function of the consumption of cold in the chamber 35, assuming the losses of vaporised nitrogen to be Zero, is as follows:

When the consumption of cold in the chamber 35 iszero, the air only has added thereto, through the pipe'3, the gaseous nitrogen separated in the rectifier column and nitrogen vapour formed in the reservoir 33. The compressor 4 compresses a mixture of air and nitrogen, with about 12% of oxygen, from which the rectifier columns produce the liquid nitrogen which is collected in the reservoir 33.

When the consumption of cold in the chamber 35 is no longer zero, the rate of flow of gaseous nitrogen sent back to the intake of the compressor increases and the rate of flow of air drawn in by the pipe 1 is reduced to the same extent. The oxygen content of the gas to be separated decreases. It is necessary to reduce the rate of flow of gas sent to the expansion device 8 in proportion to the larger input of cold into the exchanger 12. The quantity of nitrogen drawn off from the upper end of the rectification column 10 and sent to the storage reservoir 33 through the pipe 31 is thus increased. As the quantity of nitrogen separated at the upper end of the column 10 is approximately constant, it is suitable to close progressively the valve 30 for the introduction of the reflux liquid to the top of the low-pressure column 21 in proportion as the rate of flow of liquid nitrogen increases. When the quantity of liquid nitrogen vaporised in the chamber 35 becomes equal to or slightly greater than the quantity of liquid nitrogen produced (this being produced for a delivery of nitrogen which is about one-- to ensure a reflux in the low-pressure column, the rate of flow of additional air to the intake of the compressor being zero and the treated gaseous mixture being practically pure nitrogen.

If the consumption of cold in the chamber 35 increases further, the flow of gas passing through the expansion machine 8 continues to decrease, while the delivery of liquid nitrogen produced becomes smaller than the delivery of vaporised nitrogen. A certain quantity of excess gaseous nitrogen is evacuated through the pipe 1, as indicated by the broken-line arrows.

When the delivery of liquefied nitrogen becomes greater than approximately half the delivery of compressed gas, the production of liquid nitrogen becomes greater than the quantity which can be separated in the high-pressure rectifier column 10. It is then expedient progressively to open the valve 18 so as to introduce the necessary supplement of liquid nitrogen into the head of the column 10, closing at the same time the valve 19 proportionately to the increase in the production of liquid nitrogen. The apparatus then functions substantially as a nitrogen liquefier, the rectification columns no longer playing any part.

Finally, when the delivery of vaporised nitrogen in the chamber 35 becomes approximately equal to the delivery of compressed gas, the delivery of nitrogen passing through the expansion machine is completely shut ofi; the quantity of liquid nitrogen produced becomes approximately three times the normal production (for zero cold consumption). Beyond this value, the production of liquid nitrogen remains practically stationary.

It will be seen that when the installation is to be restarted after a stoppage, this can be achieved by sending only air into the compressor 4 until the rectifier columns have reached their normal running conditions and are producing gaseous nitrogen.

Referring now to FIGURE 2 of the accompanying drawing, there is shown a double rectifier column which operates in a manner similar to that of FIGURE 1, except as regards the reflux liquid nitrogen in the column at a low pressure. The latter is no longer taken from the top of the column at a high pressure, but from the middle zone of the latter through a pipe 59; the liquid nitrogen is super-cooled in a heat exchanger 27A in counter-current with the separated gaseous nitrogen, then introduced by way of a pipe 51 and an expansion valve 52 into the head of the low-pressure rectification column 21. This makes it possible to have available a higher rate of flow of the reflux liquid without causing any inconvenience. It is in fact unimportant that the gaseous nitrogen sepa rated at the head of the low-pressure rectification column 21 is not absolutely pure, since it is intended to be recycled.

Installations for the production of cold by vaporisation of liquid nitrogen, such as those shown in FIGURES 1 and 2, make it possible to conform to a consumption of cold which can vary between zero and a value several times greater than that corresponding to the vaporisation of a quantity of liquid nitrogen equal to the normal pro duction of the separation apparatus (at zero cold consumption) without the purity of the liquid nitrogen decreasirn to an appreciable degree; the latter can for example constantly remain higher than 99% Referring finally to FIGURE 3, there is shown an installation which is similar to that of FIGURES 1 and 2, but comprises only a single rectifier column. It is thus simpler than the first installation, but supplies a smaller yield of nitrogen from the liquefied gas. When the consumption of liquid nitrogen increases from zero, the oxygen content of the liquid nitrogen produced is slightly raised, i.e. by a few percent. If this is of no inconvenience from the point of view of the consumption of cold, this installation can also be arranged to conform to a variable cold consumption. Those parts of the installation which are similar to those of FIGURE 1 will not be described again. The rectifier column 10 at high pressure has arranged above it a combined condenser and vaporiser 20 surrounded by a bath of liquefied gas fed with oxygenenriched liquid coming from the sump of the column 1%) through the pipe 22 and the expansion valve 25. The nitrogen-enriched gas, after having been super-cooled in the coil 14 of the sump and the exchanger 16, in countercurrent with the separated nitrogen, is introduced as betore through the expansion valves 18 and/or 19 into the high-pressure rectification column 10. The liquid nitrogen is drawn off from the top of the column 10 through the pipe 31, expanded in the valve 32 and introduced into the reservoir 33. The gaseous nitrogen evacuated at the dome of the condenser 20 is expanded in a valve 48 to the region of atmospheric pressure, reheated in the exchanger 16 and combined by means of the pipe 44 with the nitrogen vapours coming from the device 36 for the production of cold. The oxygen-enriched gas is returned through the pipes 45 and 46 into the exchangers 12 and 6 before being discharged at 47.

What I claim is:

1. A method for the production of cold by the vaporization of liquid nitrogen according to a variable cold demand, comprising the steps of:

'(a) cooling a constant amount of a gaseous feed and separating from the cooled feed a gaseous fraction containing any oxygen present in said feed, gaseous nitrogen and liquid nitrogen;

5 (b) warming up by heat exchange with said gaseous feed said gaseous fraction, and discharging it;

(c) warming up by heat exchange with said gaseous feed said gaseous nitrogen;

(d) establishing a store of said separated liquid nitro- (e) utilizing controlled amounts of liquid nitrogen from said store to produce varying amounts of cold by vaporization of varying amounts of liquid nitrogen;

(f) warming up at least part of said vaporized nitrogen by heat exchange with said gaseous feed;

(g) mixing said warmed up gaseous nitrogen, said warmed up vaporized nitrogen and any necessary additional air amount, to make up said constant amount of said gaseous feed;

(h) decreasing the additional air amount in said gaseous feed as the amount of vaporized liquid nitrogen increases, and suppressing it when the amount of warmed up nitrogen is equal to the amount of gaseous feed;

(i) discharging any excess amount of warmed up nitro gen when said amount of warmed up nitrogen is higher than said constant amount of gaseous feed.

2. A method according to claim 1, wherein the separation of the gaseous feed is performed in two successive rectification zones under a higher pressure and under a lower pressure, in indirect heat exchange relationship, and a liquid rich in nitrogen is withdrawn from the middle part of the higher pressure rectification zone, expanded and introduced into the lower pressure rectification zone as a reflux liquid.

3. A method according to claim 1, wherein the separation of the gaseous feed is performed in two successive rectification zones under a higher pressure and under a lower pressure in indirect heat exchange relationship, a liquid rich in nitrogen is withdrawn from the upper part of the higher pressure rectification zone, expanded and introduced into the lower pressure rectification zone as a reflux liquid, and the flow rate of said liquid rich in nitrogen is decreased when the amount of vaporized liquid nitrogen increases, and the introduction of said liquid rich in nitrogen into the lower pressure rectification zone is suppressed when the amount of vaporized liquid nitrogen is at least equal to the amount of separated liquid nitrogen.

References Cited UNITED STATES PATENTS 3,062,016 11/1962 Dennis et al 6229 X 3,210,950 10/1965 Lady 6230 X FOREIGN PATENTS 623,866 6/1962 Japan.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner. 

1. A METHOD FOR THE PRODUCTION OF COLD BY THE VAPORIZATION OF LIQUID NITROGEN ACCORDING TO A VARIABLE COLD DEMAND, COMPRISING THE STEPS OF: (A) COOLING A CONSTANT AMOUNT OF A GASEOUS FEED AND SEPARATING FROM THE COOLED FEED A GASEOUS FRACTION CONTAINING ANY OXYGEN PRESENT IN SAID FEED, GASEOUS NITROGEN AND LIQUID NITROGEN; (B) WARMING UP BY HEAT EXCHANGE WITH SAID GASEOUS FEED SAID GASEOUS FRACTION, AND DISCHARGING IT; (C) WARMING UP BY HEAT EXCHANGE WITH SAID GASEOUS FEED SAID GASEOUS NITROGEN; (D) ESTABLISHING A STORE OF SAID SEPARATED LIQUID NITROGEN; (E) UTILIZING CONTROLLED AMOUNTS OF LIQUID NITROGEN FROM SAID STORE TO PRODUCE VARYING AMOUNTS OF COLD BY VAPORIZATION OF VARYING AMOUNTS OF LIQUID NITROGEN; (F) WARMING UP AT LEAST PAT OF SAID VAPORIZED NITROGEN BY HEAT EXCHANGE WITH SAID GASEOUS FEED; (G) MIXING SAID WARMED UP GASEOUS NITROGEN, SAID WARMED UP VAPORIZED NITROGEN AND ANY NECESSARY ADDITIONAL AIR AMOUNT, TO MAKE UP SAID CONSTANT AMOUNT OF SAID GASEOUS FEED; (H) DECREASING THE ADDITIONAL AIR AMOUNT IN SAID GASEOUS FEED AS THE AMOUNT OF VAPORIZED LIQUID NITROGEN INCREASES, AND SUPPRESSING IT WHEN THE AMOUNT OF WARMED UP NITROGEN IS EQUAL TO THE AMOUNT OF GASEOUS FEED; (I) DISCHARGING ANY EXCESS AMOUNT OF WARMED UP NITROGEN WHEN SAID AMOUNT OF WARMED UP NITROGEN IS HIGHER THAN SAID CONSTANT AMOUNT OF GASEOUS FEED. 